Monitoring high voltage AC, such as that found in electricity power transmission grids, is done by means of a measurement signal derived from a voltage divider which, for reasons of cost, is generally a capacitive divider constituted by a capacitive column that drops nearly all of the high voltage connected in series with a base capacitance that can only withstand a low voltage and which delivers the measurement signal. The measurement signal is a faithful image of the waveform of the high voltage AC provided that it is fed into an instrument having an input impedance that is several tens of times greater than the impedance of the base capacitance, and except for certain special circumstances when charge is trapped in the base capacitance and causes the measurement waveform voltage to change abruptly. Charge becomes trapped due to the difference between the discharge time of the capacitive column and that of the base capacitance. When there is an interruption in the high voltage AC, the same amount of charge remains trapped in the capacitive column as in the base capacitance, but the discharge rates of the two halves of the capactive voltage divider are very different and the amount of charge remaining in each half rapidly becomes very different. Unless the AC is re-established very quickly, or after a long enough period for both halves to have discharged fully, there will be more charge on the base capacitance than on the capacitive column, thereby wrongly dividing the AC at the moment it is re-established. This is because the base capacitance discharges slowly through a high impedance, giving a slowly decaying DC measurement signal with a discharge time constant of several tens of seconds.
Unless special precautions are taken, this error voltage saturates the monitoring equipment which receives the measurement signal and makes it inoperative. One known way of protecting the monitoring equipment is to insert a selective filter centered on the frequency of the high voltage AC in between the monitoring equipment and the capacitive divider. This is done each time there is a danger of trapped charge appearing, i.e. for an appreciable period of time on some occasions that the high voltage is re-established. Unfortunately, such a filter also modifies the measurment signal and considerably reduces the accuracy of measurements made for the hundred or so seconds it is inserted.
Preferred embodiments of the present invention reduce the drawbacks due to trapped charge by eliminating it very quickly without causing too much disburbance to the measurement signal.